The present invention relates to a glass substrate for a magnetic disk and a method of evaluating the same.
Information recording technology, particularly magnetic recording technology, has greatly been advanced along with enhancement of computerization technology in recent years.
One of examples of such magnetic recording technology is a hard disk drive (HDD). A hard disk drive has primary components including a magnetic disk with a disk-like substrate and a magnetic recording layer of a magnetic thin film formed on a surface of the substrate, a spindle motor operable to rotate the magnetic disk at a high speed, a magnetic head attached to an end of a swing arm for reading magnetic data from and writing magnetic data into the magnetic recording layer of the magnetic disk, and a positioner operable to move the magnetic head on the magnetic disk in a radial direction of the magnetic disk. See, e.g., JP-A-2001-243735 (Patent Document 1).
Generally, a hard disk drive has two magnetic heads for one magnetic disk because a magnetic head is arranged for each of magnetic recording layers formed on front and rear main surfaces of a magnetic disk. Heretofore, an aluminum substrate has widely been used as a substrate for a magnetic recording medium.
In recent years, however, reduction in size, reduction in thickness, and increase in recording density of magnetic disks have increased demands for a glass substrate because a glass substrate is superior to an aluminum substrate in flatness of a surface of a substrate and in strength of a substrate.
Heretofore, a glass substrate has been produced by, for example, forming glass into a disk, chamfering the disk, polishing edge surface and main surfaces, and then performing a chemical strengthening treatment on the disk for improving shock resistance and vibration resistance, as described at paragraph [0004] of JP-A-2000-076652 (Patent Document 2).
Recording layers of magnetic layers are provided on both surfaces of a glass substrate thus produced. Such a glass substrate has been used as a magnetic recording medium.
Meanwhile, in a case of a glass substrate for a magnetic disk, an inside hole is formed in the glass substrate in order to couple a spindle motor to the glass substrate. The shape of the inside hole should preferably be close to a perfect circle in order to prevent run-out during rotation or track mis-registration (TMR) caused by such run-out.
Generally, the circularity has been known as an index of the closeness to a perfect circle.
The circularity is an index that indicates the degree of deviation from a geometrically correct circle of a circular shape. According to the conventional definition, for example, according to JIS standards, the circularity of a circular shape is defined as a difference of radii of two concentric circles that interpose the circular shape therebetween with a minimized interval. See JIS B 0621-1984 (Non-Patent Document 1).
Specifically, the circularity of an inside hole formed in a substrate for a magnetic disk is defined by measuring a shape profile of the whole circumference of the inside hole and calculating a difference between a radius of a circumscribed circle with the maximum value of the profile and a radius of an inscribed circle with the smallest value of the profile.
A circumscribed circle described herein refers to a circle of the two concentric circles according to the aforementioned definition of the circularity that has a larger radius (a circle tangent to the outermost point of the profile). An inscribed circle described herein refers to a circle of the two concentric circles that has a smaller radius (a circle tangent to the innermost point of the profile).
Since glass substrates for a magnetic disk are usually mass-produced, the circularity of inner holes of glass substrates may vary. Therefore, glass substrates are produced so that such variations are within a predetermined range (dimensional tolerance).
In a case of a glass substrate for a magnetic disk, an inside hole is formed in the glass substrate in order to couple a spindle motor to the glass substrate. In another aspect, the center of the inside hole should preferably coincide with the center of a circle drawn by an outer circumferential edge of the glass substrate in order to minimize run-out or track mis-registration (TMR) during rotation.
In a case of a glass substrate for a magnetic disk, the outer circumferential edge of the glass substrate and the inside hole do not form perfect circles in practice. The outer circumferential edge of the glass substrate and the inside hole form an annular shape with irregularities. Therefore, there is required an index indicating the coincidence of centers of those shapes.
Indexes for evaluating the degree of such coincidence include the concentricity (coaxiality).
According to Japanese Industrial Standards (JIS), the concentricity is defined as follows: The degree of disagreement of axes of two mechanical parts that are arranged so as to have a common axis. The concentricity is expressed by a measured distance between several points within a specified length of two lines or two axes. When that distance is not greater than an allowance, those axes are deemed to be coaxial, coincident, or concentric (held in alignment). In the case of two axes, the distance is ½ of a maximum difference of measured values obtained when a test indicator is attached to one of the axes and swung. See JIS B 0182: 1993 Machine Tools—Test Codes—Vocabulary (Non-Patent Document 2).
In a case of a disk such as a substrate for a magnetic disk, there are two profiles for a part relating to an outside diameter (outer circumference) and a part relating to an inside diameter (inner circumference). Therefore, the concentricity is generally defined by a difference between a center of a least-square-approximated circle of the outer circumferential profile and a center of a least-square-approximated circle of the inner circumferential profile.
Since glass substrates for a magnetic disk are usually mass-produced, the concentricity of glass substrates may vary. Therefore, glass substrates are produced so that such variations are within a predetermined range (dimensional tolerance).
Furthermore, a glass substrate for a magnetic disk is formed so as to have a predetermined thickness depending upon the thickness required for the magnetic disk. In another aspect, the thickness of glass substrates should preferably be held constant in order to minimize run-out during rotation or track mis-registration (TMR) caused by such run-out.
The thickness of a glass substrate has heretofore been defined by one or more distances between both of main surfaces of the glass substrate that have been measured at one point or at a plurality of points on the glass substrate. See, e.g., JP-A 8-147691 (Patent Document 3).
Since glass substrates for a magnetic disk are usually mass-produced, the thickness of glass substrates may vary. Therefore, glass substrates are produced so that such variations are within a predetermined range (dimensional tolerance).